1. Field of the Invention
Aspects of the present invention relate to a secondary battery, and more particularly, to an electrode assembly and a secondary battery having the same that improves thermal stability significantly by including a ceramic coating layer.
2. Description of the Related Art
Generally, a secondary battery can be reused repeatedly by charging, as opposed to a disposable battery that can be used only once. The secondary battery is generally used as a main power supply of portable devices for communication, information processing and audio/video. Recently, great interest has been concentrated on the secondary battery, and the secondary battery has been developed rapidly because it has an ultra-light weight, high energy density, high output voltage, a low self-discharging rate, environment-friendliness, and a long lifetime as a power supply.
Secondary batteries are divided into nickel-hydrogen (Ni-MH) batteries and lithium ion (Li-ion) batteries according to the electrode active material. Particularly, lithium ion batteries can be also divided, according to the kind of the electrolyte used, into batteries using a liquid electrolyte, batteries using a solid polymer electrolyte or batteries using a gel phase electrolyte. In addition, lithium ion batteries may be divided into a can type and a pouch type according to a shape of a container receiving an electrode assembly.
The lithium ion battery can provide an ultra-lightweight battery because its energy density per weight is much higher than that of a disposable battery. Average voltages per cell of the lithium ion battery and average voltages of other secondary batteries, such as a NiCad battery or a nickel-hydrogen battery, are respectively 3.6V and 1.2 V. Thus, the lithium ion battery is three times more compact than other secondary batteries. In addition, the self-discharging rate of the lithium ion battery is less than 5% a month at 20° C., with corresponds to about 1/3 of the self-discharging rate of the NiCad battery or the nickel-hydrogen battery. The lithium ion battery is environment-friendly because it does not use heavy metals such as cadmium (Cd) or mercury (Hg), and has an advantage in that it is rechargeable more than 1000 times under normal conditions. Thus, the lithium ion battery has been developed rapidly to keep pace with recent developments in information and communication technologies due to the advantages as described above
In conventional secondary batteries, a bare cell is formed by providing an electrode assembly including a cathode plate, an anode plate and a separator in a can made of aluminum or aluminum alloy, finishing an upper opening of the can with a cap assembly, and injecting an electrolytic solution into the can, and sealing the can.
The separator is a polyolefin type film separator that is provided to prevent an electrical short between the cathode and anode plates. In addition, the separator itself functions as a safety device preventing overheating of the battery. In particular, micro-holes of the separator are closed when the battery temperature is suddenly increased for any reason, such as, for example, external heat transfer. However, the separator may be damaged by an increase in battery temperature that continues for a prolonged period of time.
Additionally, if the battery temperature is not decreased by a current shutdown, the separator may be continuously melted by already generated heat, even though micro-holes of the separator are closed, when large current flows in the secondary battery in a short time due to the high capacity of the battery. Thus, a possibility of an electrical short caused by damage of the separator is increased
To solve the thermal problem of the film separator, there has been proposed a method of improving protection against internal shorts by forming a ceramic coating layer on an electrode by coating a paste formed of ceramic powder, a binder and a solvent onto the electrode.
However, to achieve the desired thermal stability of the secondary battery as described above it is desirable that the ceramic coating layer should be uniformly coated without defects. In other words, it is desirable that the ceramic powder be coated to a uniform thickness on the electrode active material without defects such as uncoated parts, pin holes and cracks.
If there are the uncoated parts, pin holes and cracks in the ceramic coating layer, current may be concentrated to the defective portion and the internal short may be generated. The secondary battery may be catch on fire or explode due to thermal decomposition of the electrode active material and decomposition of the electrolytic solution by initial heat generated by a short.